Mozzarella cheese is one of the more popular cheeses, especially for use in Italian cooking. There are generally two types of mozzarella cheese: a low-moisture mozzarella and a high-moisture mozzarella. Low moisture mozzarella, which typically has a moisture content of less than 50%, has a long shelf life and is suitable for lengthy distribution supply chains and subsequent store display. High-moisture mozzarella, such as a fresh mozzarella cheese, on the other hand, typically has a moisture content of greater than 50%. The higher moisture gives the cheese a softer and more desirable taste and texture. To maintain this desired taste and texture, fresh mozzarella cheese is often packed in water to maintain its freshness.
Unfortunately, high-moisture, water-packed fresh mozzarella cheeses are more perishable and have shorter shelf lives. The higher moisture content of the cheese and the added pack water renders the product more susceptible to microbiological growth. Fresh mozzarella cheese also naturally has a pH of about 5.8, which may further cause problems with extended freshness. In combination, the relatively high pH and high moisture content poses a risk of growth of pathogenic bacteria such as Listeria monocytogenes if contaminated with such bacteria. In addition, the typical shelf life of commercial fresh mozzarella cheese is generally only about four weeks due to gas formation by gas-producing spoilage bacteria such as bacteria from the Leuconostoc species in the event of contamination.
Whey is a diary processing byproduct from the manufacture of cheese. It is the serum or watery part of milk that is separated from the curd during the cheese-making process. Whey is often characterized by the type of cheese produced. For example, sweet whey is a whey generated from the manufacture of cheddar, mozzarella, or Swiss cheeses. On the other hand, acid whey is a whey generated from the manufacture of ricotta, impastata, cottage, or cream cheeses. Acid whey typically contains mainly lactose and low levels of denatured and highly cross-linked whey proteins. It has very limited commercial value due to difficulties in recovering such solid substances. Processing acid whey by traditional methods such as spray drying for protein and lactose recoveries is quite difficult and cost prohibitive. More often, a manufacturer simply disposes of the acid whey byproduct, and generally pays a disposal fee to get rid of it.
Nisin is a peptide-like antibacterial substance produced by microorganisms such as Lactococcus lactis subsp. lactis (formerly known as Streptococcus lactis). It has been used to help stabilize various food products and its structure is illustrated in U.S. Pat. No. 5,527,505 to Yamauchi et al. The highest activity preparations of nisin contain about 40 million International Units (IU) per gram. Commercial preparations of nisin are available. For example, one commercial preparation, NISAPLIN®, containing about 1 million IU, nisin per gram, is available from Aplin & Barrett Ltd., Trowbridge, England. Another commercial preparation, CHRISIN®, also containing about 1 million IU, nisin per gram, is available from Chr. Hanson A/S (Denmark). Nisin has no known toxic effects in humans. It is widely used in a variety of prepared dairy foods. Experimental use in preserving other foods has also been reported. Details on these applications are provided below.
In U.S. Pat. No. 5,527,505, yogurt was produced from raw milk by incorporating a nisin-producing strain, Lactococcus lactis subsp. lactis, along with the traditional yogurt culture consisting of Streptococcus salivarius subsp. thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). This patent teaches that the lactococci are needed to secrete the nisin, whose effect is to retard the activity of ST and LB. The resulting yogurt therefore contains the lactococci used to produce the nisin.
In U.S. Pat. No. 5,015,487, the use of nisin, as a representative of the class of lanthionine bacteriocins, to control undesirable microorganisms in heat processed meats is disclosed. In tests involving dipping frankfurters in nisin solutions, the growth of L. monocytogenes was effectively inhibited upon storage at 40° F.
Chung et al. (Appl. Envir. Microbiol., 55, 1329-1333 (1989)) report that nisin has an inhibitory effect on gram-positive bacteria, such as L. monocytogenes, Staphylococcus aureus, and Streptococcus lactis, but has no such effect on gram-negative bacteria such as Serratia marcescens, Salmonella typhimurium, and Pseudomonas aeruginosa. 
Nisin or a nisin-producing bacterial culture has been added to cheeses to inhibit toxin production by Clostridium botulinum (U.S. Pat. No. 4,584,199). Nisaplin® has been found to preserve salad dressings from microbiological contamination for extended shelf life periods (Muriana et al., J. Food Protection, 58:1109-1113 (1995) (challenge studies using Lactobacillus brevis subsp. lindnen)).
More recently, whey from nisin-producing cultures has been used to preserve and stabilize food compositions, including fermented dairy products, mayonnaise-type spreads, cream cheese products, meat compositions, meat/vegetable compositions, and cooked pasta. These uses of whey from nisin-producing cultures are described in U.S. Pat. Nos. 6,136,351 (“Stabilization of Fermented Dairy Compositions Using Whey from Nisin-Producing Cultures”); 6,113,954 (“Stabilization of Mayonnaise Spreads Using Whey from Nisin-Producing Cultures”); 6,110,509 (“Stabilization of Cream Cheese Compositions Using Nisin-Producing Cultures”); 6,242,017 (“Stabilization of Cooked Meat Compositions Stabilized by Nisin-Containing Whey and Methods of Making”); and 6,613,364 (“Stabilization of Cooked Meat and Vegetable Compositions Using Whey From Nisin-Producing Cultures and Product Thereof”); and U.S. patent application Ser. No. 09/779,756 now U.S. Pat. No. 6,797,308 (“Stabilization of Cooked Pasta Compositions Using Whey From Nisin-Producing Cultures”). These applications, which are owned by the same assignee as the present invention, are incorporated herein by reference in their entireties.
Methods of producing the nisin-containing whey have also been documented. For example, a method of producing nisin-containing whey from skim milk is disclosed in U.S. Pat. Nos. 5,716,811; 6,242,017; 6,110,509; 6,136,351; and 6,113,954. Furthermore, another method to produce a similar nisin-containing whey from sweet whey is disclosed in U.S. Pat. No. 6,613,364 and U.S. patent application Ser. No. 09/779,756 now U.S. Pat. No. 6,797,308.
Unfortunately, a shortcoming of the existing nisin-containing whey compositions and accompanying methods for their production is that the inhibitor is unsuitable for use in fresh mozzarella cheese and the accompanying pack water. The poor clarity of existing nisin-containing whey compositions manufactured using known methods is unacceptable for addition to the clear pack water of fresh mozzarella cheese because it renders the product undesirable to consumers. Moreover, the resulting acidity of nisin-containing whey compositions made from known methods actually shortens the shelf-life of fresh mozzarella cheese by affecting the texture and integrity of the cheese.
Accordingly, there remains a need to provide a stabilized, fresh mozzarella cheese and a need to provide a method of producing an antimicrobial ingredient that is suitable for use in the pack water of fresh mozzarella cheese. In particular, there remains a need to improve the safety of fresh mozzarella cheese by retarding the growth of pathogenic bacteria or limiting their growth below detection levels and there also remains a need to increase the usable shelf life of a fresh mozzarella cheese in pack water by retarding or limiting below detection levels gas-forming bacteria. There also remains a need to provide a stabilized fresh mozzarella cheese using natural and innocuous ingredients. Furthermore, there also remains a need to provide a value added use to the acid whey byproduct from cheese manufacture.